The sickly green radiation hit effect will not appear on craft that have never had armor in their configuration, although they will still take damage. Adding a single armor plate to be destroyed makes the effect visible.
Radiation damage (with or without the graphical effect) gets carried through the shield on unarmored craft (either by design or through fighter attacks, ect) only, on an approximate 20%(?) of shots. Rad cannon only.
In both cases the presence of armor is a binary concern, you either have it or you don’t. Strength doesn’t seem to matter.
Given that the little red armor flashes play occasionally on still-shielded craft when under attack, perhaps the two are somehow related?